The present invention relates to a condenser microphone, and more particularly to an electret microphone used for a portable telephone, video camera and others.
A conventional electret microphone comprises a microphone part and a case part storing the microphone part. The microphone part is composed of metal except for a substrate made of plastic, and the case part is mainly composed of metal. However, such a metal constitution has disadvantages in processing and assembly accuracy. Consequently, it is difficult to manufacture an electret microphone having a small size and high performance.
Japanese Patent Application Laid Open 2000-50393 discloses an electret microphone mainly composed of ceramic.
FIG. 4 is a sectional view showing the electret microphone. The electret microphone comprises a microphone part 100 and a case part 200.
The case part 200 comprises a substrate 210 made of insulation material, a first frame 220, second frame 230, third frame 240, fourth frame 250, and a cover 260, which frames and cover are stacked on the substrate 210 and adhered to each other. The first, second and third frames 220, 230 and 240 are made of ceramic, and the fourth frame 250 is made of metal.
As shown in FIG. 5, each of the frames 220, 230, 240 and 250 has a square shape. On the substrate 210 and first to third frames 220-240, connecting electrodes 210b, 220b, 230b and 240b are provided by conductive films, and these electrodes are contacted with each other. Outside sizes of the frames are same, but inside size of the third frame 240 is larger than that of the first and second frames 220 and 230, and the inside size of the fourth frame 250 is larger than that of the third frame 240. Thus, a first shoulder 230a and a second shoulder 240a are formed on the second frame 230 and on the third frame 240.
Referring to FIG. 4, the microphone part 100 comprises a back electrode 110 made of metal and secured to the first shoulder 230a, an electret layer 120 formed on the back electrode 110, a diaphragm electrode 140 mounted on the third frame 240 interposing a lower spacer 150, and an upper spacer 160 between the diaphragm electrode 140 and the cover 260.
The diaphragm electrode 140 and the back electrode 110 compose a condenser. The diaphragm electrode 140 is vibrated by air entering passing through a sound collecting hole 260a of the cover 260. The capacitance of the condenser changes with the vibration of the diaphragm electrode 140 to generate an electric signal. The electric signal is transmitted to an integrated circuit 170 on the substrate 210 through connecting electrodes 210b, 220b and 240b. 
The electret microphone can be manufactured with high accuracy, because the frames are made of ceramic.
However, the back electrode 110, diaphragm electrode 140 and the fourth frame 250 are made of metal. Therefore, there are problems about temperature characteristic and others based on differences in manufacturing accuracy and coefficient of thermal expansion.
Furthermore, it is difficult to miniaturize the microphone, because of double construction that the back electrode 110 and the diaphragm electrode 140 are assembled in the case part 200 comprising the first frame 220, second frame 230, third frame 240, and fourth frame 250.